Generally, semiconductor devices are used in a variety of electronic applications, such as computers, cellular phones, personal computing devices, and many other applications. For example, home, industrial, and automotive devices that, in the past, comprised only mechanical components now have electronic parts that require semiconductor devices.
Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semi-conductive and insulating materials that are patterned and etched to form integrated circuits (ICs). There may be a plurality of transistors, memory devices, switches, conductive lines, diodes, capacitors, logic circuits, and other electronic components formed on a single die or chip, for example.
The accurate reproduction of patterns on the surface of a semiconductor substrate is critical to the proper fabrication of semiconductor devices. The semiconductor substrate may have undergone previous fabrication processes and may already feature layers and structures created by those fabrication processes. Improperly reproduced patterns can result in semiconductor devices that do not operate to design specifications or that do not operate at all. For example, transistors can be created with improperly sized gates; conductors can be created that are short circuited or open circuited with other conductors or devices; structures can be created with wrong geometries, and so forth. Improperly reproduced patterns can reduce the yield of the fabrication process, thereby increasing the overall cost of the product. The reproduction process typically involves the use of optical lithography to reproduce the patterns onto the surface of the semiconductor substrate followed by a variety of processes either to subtract (for example, etch) or to add (for example, deposit) materials from and to the semiconductor substrate.
There is a trend in the semiconductor industry towards reducing the size of features, e.g., the circuits, elements, conductive lines, and vias of semiconductor devices, in order to increase performance of the semiconductor devices, for example. The minimum feature size of semiconductor devices has steadily decreased over time. However, as features of semiconductor devices become smaller, it becomes more difficult to pattern the various material layers because of diffraction and other effects that occur during a lithography process. Interference and processing effects can cause distortion and deviation in the mask's patterns as they are reproduced onto the semiconductor substrate. For example, key metrics such as resolution and depth of focus of the imaging systems may suffer when patterning features at small dimensions.
Shrinking device geometries have a particularly acute affect on patterning small contact holes. As contact holes become smaller, masks used to pattern contact holes require smaller apertures. The smaller the aperture, the more difficult it is to get enough light through the aperture to adequately expose the resist disposed on the semiconductor wafer below. If enough light is not used to expose the mask, a pattern will not print on the resist below. If, on the other hand, the mask and semiconductor wafer is exposed with a high intensity of light, the exposed pattern below becomes washed out and side-lobes are exposed beyond the desired perimeter of the exposed area. Consequently, the generation and exposure of small contact whole regions require a very narrow lithographic process window, thereby limiting the range of exposure settings that will produce an adequate exposure. These exposure settings can include illumination or dose, focus, numerical aperture, and light coherence factor, sigma.
Hence, what is needed are methods, designs and structures of producing small geometry contact holes without degrading manufacturing process windows.